Manufacturing method of group of whiskers

ABSTRACT

A seed substrate is placed to face a formation substrate, and then a gas containing silicon is introduced and chemical vapor deposition is performed. There is no particular limitation on a kind of a material used for the formation substrate as long as the material can withstand the temperature at which the reduced pressure chemical vapor deposition is performed. A group of silicon whiskers which does not include a seed atom can be grown directly on and in contact with the formation substrate. Further, the substrate provided with the group of whiskers can be applied to a solar cell, a lithium ion secondary battery, and the like, by utilizing surface characteristics of the group of whiskers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a manufacturing method of a group ofwhiskers.

2. Description of the Related Art

In recent years, solar photovoltaic power generation has attractedattention in the new energy field. The solar photovoltaic powergeneration is a method of converting energy of sunlight into electricalenergy, and solar photovoltaic power is one of renewable energies. Theadvantages of the solar photovoltaic power generation are that there isno worry about depletion of a raw material and that greenhouse effectgases are not generated while power generation. As for solarphotovoltaic power generation, a solar cell for which single crystalsilicon, polycrystalline silicon, amorphous silicon, or the like is usedhas been put to practical use and widely used. Nowadays, in order toimprove solar cell characteristics, a technique for improving theconversion efficiency has been developed. Patent Document 1 discloses amanufacturing method of a solar cell which includes a silicon nanowireand is designed to prevent reflection of sunlight and achieve aconversion efficiency of 35% that is upper limit of crystallinesilicon-based solar cell.

Patent Document 2 discloses a technique in that a large number of minuteneedle-like silicon crystals are grown on one surface side of asubstrate, which are applied to a photoelectric conversion device suchas a solar cell or a power storage device such as a secondary batterywith ion mobility.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2010-192870-   [Patent Document 2] Japanese Published Patent Application No.    2010-210579

SUMMARY OF THE INVENTION

When a group of whiskers is grown on a substrate, a seed atom layerformed on the substrate has a function of promoting the growth of thegroup of whiskers. A nucleus (a seed atom) is generated and at the sametime, a group of whiskers starts to grow from the nucleus so as tofollow the nucleus. In other words, the group of whiskers is grown onthe substrate provided with the seed atom layer; thus, the nucleusgenerally exists on an end of the group of whiskers, and it is extremelydifficult to grow a group of whiskers that does not include a nucleus.There is another problem in that growth of a group of whiskers directlyon a substrate provided with no seed atom layer is impossible inprinciple.

In view of the above problems, an object of one embodiment of thedisclosed invention is to provide a manufacturing method of a group ofwhiskers in which a group of whiskers that does not include a nucleus(seed atom) can be grown directly on a substrate provided with no seedatom layer.

One embodiment of the present invention is a manufacturing method of agroup of whiskers, including the steps of placing a first substrate andan insulating substrate so that a surface with a seed atom layer of thefirst substrate is substantially parallel to one surface of theinsulating substrate, and introducing a gas containing silicon andperforming chemical vapor deposition so as to grow a group of whiskers.

One embodiment of the present invention is a manufacturing method of agroup of whiskers, including the steps of: placing a substrateconstituted by a seed atom and an insulating substrate so that onesurface of the substrate constituted by the seed atom is substantiallyparallel to one surface of the insulating substrate, and introducing agas containing silicon and performing chemical vapor deposition so as togrow a group of whiskers.

One embodiment of the present invention is a manufacturing method of agroup of whiskers, including the steps of: placing a first substrate, aninsulating substrate, and a second substrate so that a surface with afirst seed atom layer of the first substrate is substantially parallelto one surface of the insulating substrate, and a surface with a secondseed atom layer of the second substrate is parallel to the other surfaceof the insulating substrate, and introducing a gas containing siliconand performing chemical vapor deposition so as to grow a group ofwhiskers.

One embodiment of the present invention is a manufacturing method of agroup of whiskers, including the steps of: placing a substrateconstituted by a first seed atom, an insulating substrate, and asubstrate constituted by a second seed atom so that a surface of thesubstrate constituted by the first seed atom is substantially parallelto one surface of the insulating substrate, and a surface of thesubstrate constituted by the second seed atom is parallel to the othersurface of the insulating substrate, and introducing a gas containingsilicon and performing chemical vapor deposition so as to grow a groupof whiskers.

In one embodiment of the present invention, a reduced pressure chemicalvapor deposition device is used for the step of introducing a gascontaining silicon and performing chemical vapor deposition so as togrow a group of whiskers.

In one embodiment of the present invention, the step of introducing agas containing silicon and performing chemical vapor deposition isperformed under conditions that temperature is greater than or equal to600° C. and less than or equal to 700° C.; pressure is greater than orequal to 20 Pa and less than or equal to 200 Pa; a flow rate of a SiH₄gas is greater than or equal to 300 sccm and less than or equal to 3000sccm, a flow rate of an N₂ gas is greater than or equal to 0 sccm andless than or equal to 1000 sccm (the flow rate of the SiH₄ gas isgreater than the flow rate of the N₂ gas); a period of time is greaterthan or equal to 120 minutes and less than or equal to 180 minutes, andgrowth of whiskers continuously proceeds utilizing the reduced pressurechemical vapor deposition device.

In one embodiment of the present invention, a distance between thesubstrates is greater than or equal to 1.0 cm and less than or equal to3.0 cm.

In one embodiment of the present invention, a thickness of the seed atomlayer is greater than or equal to 10 nm and less than or equal to 1000nm.

In one embodiment of the present invention, a single whisker of thegroup of whiskers has a width of 50 nm to 300 nm, a diameter of 100 nmto 400 nm, and a length of 700 nm to 800 nm.

In one embodiment of the present invention, the first substrate and thesecond substrate are each any one of an aluminosilicate glass substrate,a barium borosilicate glass substrate, an aluminoborosilicate glasssubstrate, a sapphire substrate, and a quartz substrate.

According to one embodiment of the present invention, a manufacturingmethod of a group of whiskers can be obtained in which a group ofwhiskers that does not include a nucleus (a seed atom) can be growndirectly on a substrate provided with no seed atom layer.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are schematic cross-sectional views illustrating anexample of a group of whiskers;

FIG. 2 is a schematic cross-sectional view illustrating an example of agroup of whiskers;

FIGS. 3A and 3B are each a SEM image showing a top surface of a group ofwhiskers;

FIGS. 4A to 4C are each a STEM image showing a cross section of a singlewhisker;

FIGS. 5A to 5D are diagrams illustrating a whisker growth mechanism;

FIGS. 6A and 6B are schematic cross-sectional views illustrating anexample of a group of whiskers; and

FIG. 7 is a schematic cross-sectional view illustrating an example of agroup of whiskers.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. Note that thepresent invention is not limited to the following description, and it iseasily understood by those skilled in the art that modes and details canbe modified in various ways. Therefore, the present invention is notconstrued as being limited to description of the embodiments describedbelow.

Note that the term a “single whisker” in this specification and the likemeans a whisker included in a group of whiskers. Further, the term a“group of whiskers” includes an aggregation of single whiskers, anaggregation of single whiskers each of which grows from a root ofanother single whisker and part or the whole thereof is combined withthe another single whisker, an aggregation of whiskers into which asingle whisker has branched and then grown, and the like.

Note that in this specification and the like, the term “over” or “below”does not necessarily mean that a component is placed “directly on” or“directly under” another component. Moreover, the terms “over” and“below” are simply used for convenience of explanation.

Note that the position, the thickness, the size, or the like of eachstructure illustrated in drawings and the like is not accuratelyrepresented in some cases for simplification. Therefore, one embodimentof the disclosed invention is not necessarily limited to the position,the thickness, the size, or the like as disclosed in the drawings andthe like.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not mean limitation of the number ofcomponents.

Embodiment 1

In this embodiment, a manufacturing method of a group of whiskersaccording to one embodiment of the disclosed invention is described.Note that in the manufacturing method of a group of whiskers describedin this embodiment, a group of silicon whiskers that does not include aseed atom can be grown directly on a substrate provided with no seedatom layer.

FIGS. 1A and 1B and FIG. 2 are cross-sectional views schematicallyillustrating examples in which a group of whiskers is grown on aformation substrate. A manufacturing method of a group of whiskers isdescribed below with reference to FIGS. 1A and 1B and FIG. 2.

As in FIG. 1A, a seed substrate 100 and a formation substrate 101 areplaced in a susceptor 106. The seed substrate 100 includes a seed atomlayer 100 a and a substrate 100 b. Note that the seed substrate 100 andthe formation substrate 101 are placed so that a surface of the seedatom layer 100 a of the seed substrate 100 is parallel to one surface ofthe formation substrate 101.

The seed atom layer 100 a is formed on the substrate 100 b by a printingmethod, a coating method, an ink-jet method, a CVD method, a sputteringmethod, an evaporation method, or the like, as appropriate.

There is no particular limitation on a material used for the seed atomlayer 100 a, as long as the material can directly (or indirectly)promote growth of a group of whiskers. For example, a metal atomtypified by titanium, nickel, tungsten, cobalt, iron, chromium, and thelike may be used, or an atom other than the metal atom may be used. Inthis embodiment, titanium is used for the seed atom layer 100 a.

As a material used for the substrate 100 b, an aluminosilicate glass, abarium borosilicate glass, an aluminoborosilicate glass, sapphire,quartz, or the like can be used. Alternatively, a substrate in which aninsulating film is formed over a metal substrate such as a stainlesssteel substrate may be used. In this embodiment, a glass substrate isused as the substrate 100 b.

Next, the substrates are put in a furnace, a gas containing silicon isintroduced, and a reduced pressure chemical vapor deposition isperformed. The deposition conditions in this embodiment are as follows:the flow rate of a SiH₄ gas (a source gas) is 300 sccm; the flow rate ofan N₂ gas is 300 sccm; the temperature in the furnace is 600° C.; thepressure in the furnace is 20 Pa; the treatment time is 135 minutes; andthe distance d between a bottom surface of the substrate 100 b and abottom surface of the formation substrate 101 (the pitch distance of thesusceptor 106, hereinafter referred to as the distance betweensubstrates) is 25.4 mm. Note that a He gas with a flow rate of 200 sccmis made flow when the temperature is increased or decreased.

The source gas may be a deposition gas containing at least silicon, andis not limited to a SiH₄ gas. Examples of the deposition gas containingsilicon include silicon hydride, silicon fluoride, and silicon chloride;typically, SiH₄, Si₂H₆, SiF₄, SiCl₄, Si₂Cl₆, and the like are given.Note that hydrogen may be introduced into the source gas.

Further, as an inert gas introduced with the source gas, an N₂ gas, anAr gas, a He gas, and the like are given. However, the inert gas is notnecessarily introduced. In this embodiment, an N₂ gas is introduced.

As the result of the introduction of a gas containing silicon and areduced pressure chemical vapor deposition, a group of whiskers 102 isformed on the formation substrate 101, as illustrated in FIG. 1B.

There is no particular limitation on a kind of a material used for theformation substrate 101 as long as the material can withstand thetemperature at which the reduced pressure chemical vapor deposition isperformed. An insulating substrate typified by a quartz substrate, analuminum oxide substrate, or the like, a silicon wafer, or the like canbe used. Note that a substrate having an insulating surface or asubstrate the whole of which is formed of an insulating material isreferred to as an insulating substrate. In this embodiment, a quartzsubstrate is used as the formation substrate 101.

Although there is no particular upper limit of the thickness of the seedatom layer 100 a, the thickness is preferably at least less than orequal to 0.5 μm. In view of productivity, the thickness is preferablyless than or equal to 0.1 μm, further preferably about 10 nm to 50 nm.The thickness of the seed atom layer 100 a has effect on formation ofthe group of whiskers 102 on the formation substrate 101. Note that whenthe thickness of the seed atom layer 100 a is about 10 nm to 50 nm, itis easier to form the group of whiskers 102 on the formation substrate101.

The degree of deposition of silicon atoms on the seed substrate 100,that is, easiness of forming a group of silicon whiskers, hasflexibility. As illustrated in FIG. 1B, a group of whiskers 104 can beformed on the seed substrate 100. Alternatively, as illustrated in FIG.2, a polysilicon layer 105 can be formed over the seed substrate 100without forming a group of whiskers. Therefore, the thickness of theseed atom layer 100 a has considerable effect on formation of the groupof whiskers on the seed substrate 100. As the thickness of the seed atomlayer 100 a is smaller, the group of whiskers is more likely to beformed on the seed substrate 100 and the formation state of the group ofwhiskers becomes closer to the state of FIG. 1B. On the other hand, asthe thickness of the seed atom layer 100 a is larger, the group ofwhiskers is less likely to be formed on the seed substrate 100, and theformation state of the group of whiskers becomes closer to the state ofFIG. 2.

Further, in FIG. 1B, when the group of whiskers 102 is compared with thegroup of whiskers 104 formed on the seed substrate 100, there are manydifferences in the density of a group of whiskers, the number of singlewhiskers, the shape and the size of a single whisker, and the likethough the group of whiskers 102 and the group of whiskers 104 areformed on the substrates in the same reduced pressure chemical vapordeposition step. The density of the group of whiskers 102 is low, singlewhiskers of the group of whiskers 102 are sparsely formed, and a singlewhisker of the group of whiskers 102 tends to have a shape with arounded corner. The density of the group of whiskers 104 is high, thelength of a single whisker of the group of whiskers 104 is longer thanthe length of a single whisker of the group of whiskers 102, and singlewhiskers of the group of whiskers 104 are formed densely and tend tohave various shapes.

Note that the seed substrate 100 according to this embodiment may beconstituted only by the seed atom layer 100 a. There is no particularlimitation on the seed substrate 100, as long as it can directly (orindirectly) promote growth of the group of whiskers. Accordingly, ametal plate such as a titanium sheet may be used, for example. Note thatit is appropriate that the thickness of the titanium sheet is about 0.1mm to 0.6 mm.

The shape of a single whisker of the group of whiskers 102 and the shapeof a single whisker of the group of whiskers 104 may be a columnar shapesuch as a cylinder shape or a prism shape, a cone shape, or a pyramidshape. Alternatively, the shape of a single whisker of the group ofwhiskers 102 and the shape of a single whisker of the group of whiskers104 may be a needle shape with a sharp end, a shape with a curved end, ashape with round corners, or a taper shape in which either or both sidesurfaces are inclined.

Note that the shapes and sizes of a single whisker of the group ofwhiskers 102 and a single whisker of the group of whiskers 104 changedepending on the melting point and the boiling point of the seed atomlayer 100 a; thus, a material of the seed atom layer 100 a is preferablyselected in accordance with the purpose of use.

A reduced pressure chemical vapor deposition method is preferablyemployed for a step for growing a group of whiskers. Note that a thermalchemical vapor deposition method is classified into two types accordingto pressure in deposition: a reduced pressure chemical vapor depositionmethod in which deposition is performed under pressure lower thanatmospheric pressure; and a normal atmospheric pressure chemical vapordeposition method in which deposition is performed under atmosphericpressure.

A group of whiskers which is more uniform and preferable can be formedby the introduction of a gas containing silicon and the reduced pressurechemical vapor deposition. In the deposition step under pressure lowerthan atmospheric pressure, the mean free path of atoms of a sourcematerial in a vapor phase is long; thus, the atoms of the sourcematerial sufficiently diffuse even in a step portion, and the atoms canreach a narrow space. Therefore, favorable step coverage can beobtained. Further, in the deposition step under pressure lower thanatmospheric pressure, the diffusion coefficient of the atoms of thesource material becomes high, and the deposition rate depends onreaction of atoms at a surface rather than transfer of atoms at thesurface. Therefore, a rate-determining region can be shifted to a highertemperature side.

Note that in the case where a metal atom which reacts with silicon toform silicide is used for the seed atom layer 100 a as in thisembodiment, silicide is formed at an interface between the seed atomlayer 100 a and silicon. A silicide 103 is formed at an interfacebetween a root portion of the group of whiskers 104 and the seed atomlayer 100 a in FIG. 1B, or at an interface between the polysilicon layer105 and the seed atom layer 100 a in FIG. 2.

For example, in the case where a metal atom typified by titanium,nickel, tungsten, cobalt, iron, chromium, or the like is used as amaterial for the seed atom layer 100 a, titanium silicide, nickelsilicide, tungsten silicide, cobalt silicide, iron silicide, chromiumsilicide, or the like is formed at an interface between the seed atomlayer 100 a and silicon.

Note that an interface between the seed atom layer 100 a and thesilicide 103 and an interface between the silicide 103 and silicon,which are formed after the reduced pressure chemical vapor deposition,are not clear.

According to the above-described manufacturing method of a group ofwhiskers, at least the group of whiskers 102 can be formed on theformation substrate 101 which is placed to face the seed substrate 100.Further, a material for the formation substrate 101 is not limited andcan be freely selected as long as the material can withstand thetemperature at which the reduced pressure chemical vapor deposition isperformed, and the group of whiskers 102 can be grown on the formationsubstrate 101. Furthermore, since the formation substrate 101 is placedto face the seed substrate 100, the group of whiskers 102 can be growndirectly on and in contact with the formation substrate 101 providedwith no seed atom layer.

Next, FIGS. 3A and 3B and FIGS. 4A to 4C show a group of whiskers and asingle whisker thereof formed on the formation substrate by employingthe above-described manufacturing method of a group of whiskers. FIGS.3A and 3B are observation photographs taken with a scanning electronmicroscope (SEM), and FIGS. 4A to 4C are observation photographs takenwith a scanning transmission electron microscope (STEM).

A quartz substrate is used as the formation substrate 101, titanium isused for the seed atom layer 100 a, and a glass substrate is used as thesubstrate 100 b. The thickness of titanium is 500 nm.

FIGS. 3A and 3B are SEM observation photographs showing an enlarged topsurface of the group of whiskers. In a SEM observation method, a beam ofelectrons scans a top surface of a sample while being focused narrowlyusing an electric field lens so that secondary electrons and reflectionelectrons generated at the top surface are detected and a microscopeimage of the top surface of the sample is obtained.

FIGS. 3A and 3B are each an enlarged microscope observation photographof a top surface of a group of whiskers formed on a quartz substrate.FIG. 3A is taken at 3000-fold magnification with an acceleration voltageof 10.0 kV, and FIG. 3B is taken at 40000-fold magnification with anacceleration voltage of 10.0 kV.

FIGS. 4A to 4C are STEM photographs showing an enlarged cross section ofa group of whiskers and an enlarged sample of a single whisker. In aSTEM observation method, a microscope photograph is obtained by scanningwith a beam of electrons as in a scanning electron microscope (SEM) andutilizing a beam of electrons transmitted through a sample as in atransmission electron microscope (TEM).

FIG. 4A is an enlarged microscope observation photograph of a crosssection of a group of whiskers formed on a quartz substrate, which istaken at 19500-fold magnification with an acceleration voltage of 200kV. FIGS. 4B and 4C are each an enlarged observation photograph of asample of a single whisker that is cut from the group of whiskers shownin FIG. 4A by rubbing a top surface of the group of whiskers with meshmade of copper, which are taken at 150000-fold magnification with anacceleration voltage of 200 kV.

It is found that a single whisker shown in FIG. 4B has a needle shapewith a sharp end and a taper shape in which both side surfaces areinclined. Further, it is found that a single whisker shown in FIG. 4Chas a shape with a curved end and a shape close to a cylinder shape.Thus, it is found that there are differences in size and shape evenbetween single whiskers of the same group of whiskers, which are formedby introduction of a gas containing silicon and performing the samereduced pressure chemical vapor deposition step on the substrates.

According to FIGS. 3A and 3B and FIGS. 4A to 4C, sizes of singlewhiskers are different in width, diameter, and length. In general, asingle whisker has a size with a width of about 50 nm to 300 nm, adiameter of about 100 nm to 400 nm, and a length of about 700 nm to 800nm.

The size of a single whisker depends on the temperature in a furnace inthe reduced pressure chemical vapor deposition step. This is because theorientation of silicon crystal grains during the growth of a group ofwhiskers largely depends on the temperature during the growth of a groupof whiskers. Thus, when the temperature in a furnace is relatively low,a group of whiskers does not grow so much, and the higher temperature ina furnace becomes, the easier the group of whiskers grow and thus thelonger the length of a single whisker tends to be.

Further, the density of a group of whiskers depends on the distancebetween substrates. In other words, in the reduced pressure chemicalvapor deposition step, when the frequency of collision of an atom underthermal motion in a vapor phase with another atom between the seedsubstrate and the formation substrate is decreased, the probability ofgrowth of a group of whiskers is increased. In the ideal case, thedensity of a group of whiskers is the highest when the distance betweensubstrates is set to a distance corresponding to the mean free path ofatoms under the pressure of the reduced pressure chemical vapordeposition. In that case, atoms can move straight between the seedsubstrate and the formation substrate. Therefore, as the distancebetween substrates is longer, the frequency of collision of an atomunder thermal motion in a vapor phase with another atom is increased,and thus the probability of growth of a group of whiskers is decreasedand the density of the group of whiskers tends to be decreased.

As described above, the size of a single whisker and the density of agroup of whiskers change depending on the distance between substratesand the temperature in a furnace (the temperature of the seed substrateand the temperature of the formation substrate) during the reducedpressure chemical vapor deposition. In order to form a desired group ofwhiskers in accordance with an application, various conditions need tobe adjusted as appropriate.

Note that the densities, the shapes, and the sizes of a group ofwhiskers and a single whisker is changed depending on a variety ofconditions such as the distance between substrates, the temperature in afurnace, the pressure in a furnace, and the treatment time; therefore,the densities, the shapes, and the sizes are not particularly limited.

A constituent (a constituent atom) of a group of whiskers formed on theformation substrate by employing the above-described manufacturingmethod of a group of whiskers is described. According to the STEMobservation results shown in FIGS. 4A to 4C, a seed atom is not seen ina root portion and ends of a group of silicon whiskers. That is, thegroup of whiskers does not include a seed atom and is constituted onlyby silicon atoms.

A growth mechanism of a general group of whiskers is described withreference to schematic cross-sectional views shown in FIGS. 5A to 5D.First, a seed atom layer 201 is formed on a substrate 200 (see FIG. 5A).Next, a gas containing silicon is introduced and reduced pressurechemical vapor deposition is performed (see FIG. 5B). A single whisker203 a and a single whisker 203 b start to grow from a seed atom 201 aand a seed atom 201 b which are formed on the substrate 200 so as tofollow the seed atom 201 a and the seed atom 201 b. The seed atom 201 aand the seed atom 201 b are adhered to ends of the single whisker 203 aand the single whisker 203 b, respectively, and promote the growth ofthe single whisker 203 a and the single whisker 203 b by pulling siliconatoms. Further, a silicide 202 is formed in a root portion of the singlewhisker 203 a and the single whisker 203 b (see FIG. 5C). Then, thegrowth of the single whisker 203 a and the single whisker 203 b isstopped, and a single whisker 204 a having an end to which the seed atom201 a is adhered and a single whisker 204 b having an end to which theseed atom 201 b is adhered are completed (see FIG. 5D).

A growth mechanism of a group of whiskers formed on the formationsubstrate by employing the above-described manufacturing method of agroup of whiskers differs from the growth mechanism of a general groupof whiskers illustrated in FIGS. 5A to 5D.

Therefore, the group of whiskers 102 and the group of whiskers 104formed on the substrates in the same reduced pressure chemical vapordeposition step differs greatly from a general group of whiskers in agrowth mechanism, in addition to the density of a group of whiskers, thenumber of single whiskers, the shape and the size of a single whisker,and the like.

Considering the above-described growth mechanism, it is extremelydifficult to grow a group of whiskers that does not include a seed atom,against the general mechanism in that the seed atom 201 a and the seedatom 201 b are left on ends of the single whisker 204 a and the singlewhisker 204 b, respectively, and the silicide 202 is formed in a rootportion of the single whisker 204 a and the single whisker 204 b.

However, according to the above-described manufacturing method of agroup of whiskers, a group of whiskers that does not include a nucleus(a seed atom) can be grown directly on a substrate provided with no seedatom layer, against the general mechanism. When the area of a surfacewhere a polysilicon layer is formed is compared with that of a surfacewhere a group of whiskers is formed by the above-described manufacturingmethod, the surface area of the latter is significantly larger. Thus,with characteristics of such a surface, a group of whiskers can be usedfor various applications. For example, the group of whiskers is appliedto a solar cell, the power generation efficiency per unit area can beincreased and the energy cost can be reduced. Besides, the substrate onwhich the group of whiskers is formed can be applied to a lithium ionsecondary battery, an electrode material, a filter, or the like, andthus can be used for a wide variety of applications.

Embodiment 2

In this embodiment, another manufacturing method of a group of whiskersaccording to one embodiment of the disclosed invention is described.Note that in the manufacturing method of a group of whiskers describedin this embodiment, a group of silicon whiskers that does not include aseed atom can be grown directly on a substrate provided with no seedatom layer.

FIGS. 6A and 6B and FIG. 7 are cross-sectional views schematicallyillustrating examples in which a group of whiskers is grown on aformation substrate. A manufacturing method of a group of whiskers isdescribed below with reference to FIGS. 6A and 6B and FIG. 7.

As in FIG. 6A, a first seed substrate 300, a second seed substrate 305,and a formation substrate 301 are placed in a susceptor 311. The firstseed substrate 300 includes a seed atom layer 300 a and a substrate 300b. The second seed substrate 305 includes a seed atom layer 305 a and asubstrate 305 b.

The first seed substrate 300, the second seed substrate 305, and theformation substrate 301 are placed so that a surface of the seed atomlayer 300 a of the first seed substrate 300 is parallel to one surfaceof the formation substrate 301 and a surface of the seed atom layer 305a of the second seed substrate 305 is parallel to the other surface ofthe formation substrate 301.

Note that the thickness of the seed atom layer 300 a, the thickness ofthe seed atom layer 305 a, the distance d1 between a bottom surface ofthe substrate 305 b and a bottom surface of the formation substrate 301,the distance d2 between a bottom surface of the substrate 300 b and abottom surface of the formation substrate 301, and the like illustratedin FIGS. 6A and 6B and FIG. 7 do not accurately show the actualposition, thickness, size, and the like. Therefore, embodiments of thepresent invention are not necessarily limited to the position,thickness, size, and the like illustrated in drawings.

The distance d1 between the bottom surface of the substrate 305 b andthe bottom surface of the formation substrate 301, and the distance d2between the bottom surface of the substrate 300 b and the bottom surfaceof the formation substrate 301 are each the pitch distance of thesusceptor 311. The distance d1 and the distance d2 are equal to eachother.

The seed atom layer 300 a and the seed atom layer 305 a are formed onthe substrate 300 b and the substrate 305 b, respectively, by a printingmethod, a coating method, an ink-jet method, a CVD method, a sputteringmethod, an evaporation method, or the like, as appropriate.

There is no particular limitation on materials used for the seed atomlayer 300 a and the seed atom layer 305 a, as long as the materials candirectly (or indirectly) promote growth of a group of whiskers. Forexample, a metal atom typified by titanium, nickel, tungsten, cobalt,iron, chromium, and the like may be used, or an atom other than themetal atom may be used.

As materials used for the substrate 300 b and the substrate 305 b, analuminosilicate glass, a barium borosilicate glass, analuminoborosilicate glass, sapphire, quartz, or the like can be used.Alternatively, a substrate in which an insulating film is formed over ametal substrate such as a stainless steel substrate may be used.

Next, the substrates are put in a furnace, a gas containing silicon isintroduced, and a reduced pressure chemical vapor deposition isperformed. The deposition conditions in this embodiment are as follows:the flow rate of a SiH₄ gas (a source gas) is 300 sccm; the flow rate ofan N₂ gas is 300 sccm; the temperature in the furnace is 600° C.; thepressure in the furnace is 20 Pa; the treatment time is 135 minutes; andthe distance d1 between the bottom surface of the substrate 305 b andthe bottom surface of the formation substrate 301 and the distance d2between the bottom surface of the substrate 300 b and the bottom surfaceof the formation substrate 301 are equally 12.7 mm. Note that a He gaswith a flow rate of 200 sccm is made flow when the temperature isincreased or decreased.

The source gas may be a deposition gas containing at least silicon, andis not limited to a SiH₄ gas. Examples of the deposition gas containingsilicon include silicon hydride, silicon fluoride, and silicon chloride;typically, SiH₄, Si₂H₆, SiF₄, SiCl₄, Si₂Cl₆, and the like are given.Note that hydrogen may be introduced into the source gas.

Further, as an inert gas introduced with the source gas, an N₂ gas, anAr gas, a He gas, and the like are given. However, the inert gas is notnecessarily introduced. In this embodiment, an N₂ gas is introduced.

As the result of the introduction of a gas containing silicon and thereduced pressure chemical vapor deposition, a group of whiskers 302 anda group of whiskers 306 are formed on the formation substrate 301, asillustrated in FIG. 6B.

There is no particular limitation on a kind of a material used for theformation substrate 301 as long as the material can withstand thetemperature at which the reduced pressure chemical vapor deposition isperformed. An insulating substrate typified by a quartz substrate, analuminum oxide substrate, or the like, a silicon wafer, or the like canbe used. Note that a substrate having an insulating surface or asubstrate the whole of which is formed of an insulating material isreferred to as an insulating substrate.

Although there is no particular upper limit of the thicknesses of theseed atom layer 300 a and the seed atom layer 305 a, the thickness ispreferably at least less than or equal to 0.5 μm. In view ofproductivity, the thicknesses is preferably less than or equal to 0.1μm, further preferably about 10 nm to 50 nm. The thicknesses of the seedatom layer 300 a and the seed atom layer 305 a have effect on formationof the group of whiskers 302 and the group of whiskers 306 formed on theformation substrate 301. Note that when the thicknesses of the seed atomlayer 300 a and the seed atom layer 305 a are each about 10 nm to 50 nm,it is easier to form the group of whiskers 302 and the group of whiskers306 on the formation substrate 301.

The degree of deposition of silicon atoms on the first seed substrate300 and the second seed substrate 305, that is, easiness of forming agroup of silicon whiskers, has flexibility. As illustrated in FIG. 6B, agroup of whiskers 304 and a group of whiskers 307 can be formed on thefirst seed substrate 300 and the second seed substrate 305.Alternatively, as illustrated in FIG. 7, a polysilicon layer 308 and apolysilicon layer 309 can be formed on the first seed substrate 300 andthe second seed substrate 305, respectively, without forming a group ofwhiskers. Therefore, the thicknesses of the seed atom layer 300 a andthe seed atom layer 305 a have considerable effect on formation of thegroups of whiskers formed on the first seed substrate 300 and the secondseed substrate 305. As the thicknesses of the seed atom layer 300 a andthe seed atom layer 305 a are smaller, the groups of whiskers is morelikely to be formed on the seed substrate 300 and the second seedsubstrate 305 and the formation state of the group of whiskers becomescloser to the state of FIG. 6B. On the other hand, as the thicknesses ofthe seed atom layer 300 a and the seed atom layer 305 a are larger, thegroups of whiskers is less likely to be formed on the first seedsubstrate 300 and the second seed substrate 305, and the formation stateof the group of whiskers becomes closer to the state of FIG. 7.

Further, in FIG. 6B, when the group of whiskers 304 formed on the firstseed substrate 300 and the group of whiskers 307 formed on the secondseed substrate 305 are compared with the group of whiskers 302 and 306formed on the seed substrate 301, there are many differences in thedensity of a group of whiskers, the number of single whiskers, the shapeand the size of a single whisker, and the like though the groups ofwhiskers are formed on the substrates in the same reduced pressurechemical vapor deposition step. The density of the group of whiskers 302and the density of the group of whiskers 306 are each low, singlewhiskers of the group of whiskers 302 are sparsely formed and singlewhiskers of the group of whiskers 306 are also sparsely formed, and asingle whisker of the group of whiskers 302 and a single whisker of thegroup of whiskers 306 each tend to have a shape with a rounded corner.The density of the group of whiskers 304 and the density of the group ofwhiskers 307 are each high, the length of a single whisker of the groupof whiskers 304 and the length of a single whisker of the group ofwhiskers 307 are longer than that of a single whisker of the group ofwhiskers 302 and that of a single whisker of the group of whiskers 306,and single whiskers of the group of whiskers 304 and single whiskers ofthe group of whiskers 307 are formed densely and tend to have variousshapes.

Note that the first seed substrate 300 according to this embodiment maybe constituted only by the seed atom layer 300 a and the second seedsubstrate 305 according to this embodiment may be constituted only bythe seed atom layer 305 a. There are no particular limitations on theseed substrates, as long as they can directly (or indirectly) promotegrowth of the group of whiskers. Accordingly, a metal plate such as atitanium sheet may be used, for example. Note that it is appropriatethat the thickness of the titanium sheet is about 0.1 mm to 0.6 mm.

The shape of a single whisker of the group of whiskers 302, the shape ofa single whisker of the group of whiskers 304, the shape of a singlewhisker of the group of whiskers 306, and the shape of a single whiskerof the group of whiskers 307 may be a columnar shape such as a cylindershape or a prism shape, a cone shape, or a pyramid shape. Alternatively,the shapes thereof may be a needle shape with a sharp end, a shape witha curved end, a shape with round corners, or a taper shape in whicheither or both side surfaces are inclined.

Note that the shapes and sizes of a single whisker of the group ofwhiskers 302, a single whisker of the group of whiskers 304, a singlewhisker of the group of whiskers 306, and a single whisker of the groupof whiskers 307 change depending on the melting points and the boilingpoints of the seed atom layers 300 a and 305 a; thus, materials of theseed atom layers 300 a and 305 a are preferably selected in accordancewith the purpose of use.

Note that a reduced pressure chemical vapor deposition method ispreferably employed for a step for growing a group of whiskers.

A group of whiskers which is more uniform and preferable can be formedby the introduction of a gas containing silicon and the reduced pressurechemical vapor deposition. In the deposition step under pressure lowerthan atmospheric pressure, the mean free path of atoms of a sourcematerial in a vapor phase is long; thus, the atoms of the sourcematerial sufficiently diffuse even in a step portion, and the atoms canreach a narrow space. Therefore, favorable step coverage can beobtained. Further, in the deposition step under pressure lower thanatmospheric pressure, the diffusion coefficient of the atoms of thesource material becomes high, and the deposition rate depends onreaction of atoms at a surface rather than transfer of atoms at thesurface. Therefore, a rate-determining region can be shifted to a highertemperature side.

Note that in the case where a metal atom which reacts with silicon toform silicide is used for the seed atom layer 300 a and the seed atomlayer 305 a, silicide is formed at an interface between the seed atomlayer 300 a and silicon and at an interface between the seed atom layer305 a and silicon. In FIG. 6B, a silicide 303 is formed at an interfacebetween a root portion of the group of whiskers 304 and the seed atomlayer 300 a, and a silicide 310 is formed at an interface between a rootportion of the group of whiskers 307 and the seed atom layer 305 a. InFIG. 7, the silicide 303 is formed at an interface between thepolysilicon layer 308 and the seed atom layer 300 a and the silicide 310is formed at an interface between the polysilicon layer 309 and the seedatom layer 305 a.

For example, in the case where a metal atom typified by titanium,nickel, tungsten, cobalt, iron, chromium, or the like is used asmaterials for the seed atom layer 300 a and the seed atom layer 305 a,titanium silicide, nickel silicide, tungsten silicide, cobalt silicide,iron silicide, chromium silicide, or the like is formed at an interfacebetween the seed atom layer 300 a and silicon and at an interfacebetween the seed atom layer 305 a and silicon.

Note that an interface between the seed atom layer 300 a and thesilicide 303, an interface between the silicide 303 and silicon, aninterface between the seed atom layer 305 a and the silicide 310, and aninterface between the silicide 310 and silicon, which are formed afterthe reduced pressure chemical vapor deposition, are not clear.

According to the above-described manufacturing method of a group ofwhiskers, the group of whiskers 302 can be formed on one surface of theformation substrate 301 and the group of whiskers 306 can be formed onthe other surface of the formation substrate 301. Further, a materialfor the formation substrate 301 is not limited and can be freelyselected as long as the material can withstand the temperature at whichthe reduced pressure chemical vapor deposition is performed, and thegroup of whiskers 302 and the group of whiskers 306 can be grown on theformation substrate 301. Furthermore, since the formation substrate 301is placed between the first seed substrate 300 and the second seedsubstrate 305, the group of whiskers 302 and the group of whiskers 306can be grown directly on and in contact with the surfaces of theformation substrate 301 provided with no seed atom layer.

A growth mechanism of a group of whiskers formed on the formationsubstrate by employing the above-described manufacturing method of agroup of whiskers differs from the growth mechanism of a general groupof whiskers illustrated in FIGS. 5A to 5D. Therefore, the group ofwhiskers 302 and the group of whiskers 306 are constituted by onlysilicon atoms and do not include seed atoms.

Therefore, according to the above-described manufacturing method of agroup of whiskers, a group of whiskers that does not include a nucleus(a seed atom) can be grown directly on a substrate provided with no seedatom layer, against the general mechanism in that seed atoms are left onends of a group of whiskers and a silicide is formed in a root portionof the group of whiskers. When the surface area of a surface where apolysilicon layer is formed is compared with that of a surface where agroup of whiskers is formed by the above-described manufacturing method,the surface area of the latter is significantly larger. Thus, withcharacteristics of such a surface, the group of whiskers can be used forvarious applications. For example, the group of whiskers is applied to asolar cell, the power generation efficiency per unit area can beincreased and the energy cost can be reduced. Besides, the substrate onwhich the group of whiskers is formed can be applied to a lithium ionsecondary battery, an electrode material, a filter, or the like, andthus can be used for a wide variety of applications.

This application is based on Japanese Patent Application serial no.2010-232927 filed with Japan Patent Office on Oct. 15, 2010, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A manufacturing method comprising the steps of:providing a seed atom layer on a surface of a first substrate; placingthe first substrate and a second substrate so that the surface of thefirst substrate faces a surface of the second substrate, and introducinga gas containing silicon and performing chemical vapor deposition so asto grow a group of whiskers from the second substrate, wherein no seedatom layer is included with the second substrate.
 2. The manufacturingmethod according to claim 1, wherein the group of whiskers is formedfrom the second substrate in a direction to the first substrate.
 3. Themanufacturing method according to claim 1, wherein the chemical vapordeposition is reduced pressure chemical vapor deposition.
 4. Themanufacturing method according to claim 1, wherein the step ofperforming the chemical vapor deposition is performed under conditionsthat temperature is greater than or equal to 600° C. and less than orequal to 700° C., pressure is greater than or equal to 20 Pa and lessthan or equal to 200 Pa, a flow rate of a SiH₄ gas is greater than orequal to 300 sccm and less than or equal to 3000 sccm, and a flow rateof an N₂ gas is greater than or equal to 0 seem and less than or equalto 1000 sccm.
 5. The manufacturing method according to claim 1, whereina single whisker of the group of whiskers has a width of 50 nm to 300nm, a diameter of 100 nm to 400 nm, and a length of less than 800 nm. 6.The manufacturing method according to claim 1, wherein the secondsubstrate is any one of an aluminosilicate glass substrate, a bariumborosilicate glass substrate, an aluminoborosilicate glass substrate, asapphire substrate, and a quartz substrate, wherein a group of siliconwhiskers is grown directly on the second substrate.
 7. The manufacturingmethod according to claim 1, wherein a distance between the secondsubstrate and the first substrate is greater than or equal to 1.0 cm andless than or equal to 3.0 cm.
 8. The manufacturing method according toclaim 1, wherein a thickness of the seed atom layer is greater than orequal to 10 nm and less than or equal to 1000 nm.
 9. A manufacturingmethod, comprising the steps of: placing a first substrate and a secondsubstrate, the first substrate comprising a seed atom, wherein a surfaceof the first substrate faces a surface of the second substrate, andintroducing a gas containing silicon and performing chemical vapordeposition so as to grow a group of whiskers from the second substrate,wherein no seed atom layer is included with the second substrate. 10.The manufacturing method according to claim 9, wherein the group ofwhiskers is formed from the second substrate in a direction to the firstsubstrate.
 11. The manufacturing method according to claim 9, whereinthe chemical vapor deposition is reduced pressure chemical vapordeposition.
 12. The manufacturing method according to claim 9, whereinthe step of performing the chemical vapor deposition is performed underconditions that temperature is greater than or equal to 600° C. and lessthan or equal to 700° C., pressure is greater than or equal to 20 Pa andless than or equal to 200 Pa, a flow rate of a SiH₄ gas is greater thanor equal to 300 sccm and less than or equal to 3000 sccm, and a flowrate of an N₂ gas is greater than or equal to 0 sccm and less than orequal to 1000 seem.
 13. The manufacturing method according to claim 9,wherein a single whisker of the group of whiskers has a width of 50 nmto 300 nm, a diameter of 100 nm to 400 nm, and a length of less than 800nm.
 14. The manufacturing method according to claim 9, wherein adistance between the second substrate and the first substrate is greaterthan or equal to 1.0 cm and less than or equal to 3.0 cm.
 15. Amanufacturing method comprising the steps of: providing a first seedatom layer on a surface of a first substrate; providing a second seedatom layer on a surface of a third substrate; placing the firstsubstrate, a second substrate, and the third substrate so that thesurface of the first substrate faces one surface of the secondsubstrate, and the surface of the third substrate faces the othersurface of the second substrate, and introducing a gas containingsilicon and performing chemical vapor deposition so as to grow a groupof whiskers from the second substrate, wherein no seed atom layer isincluded with the second substrate.
 16. The manufacturing methodaccording to claim 15, wherein the group of whiskers is formed on theone surface and the other surface of the second substrate.
 17. Themanufacturing method according to claim 15, wherein the chemical vapordeposition is reduced pressure chemical vapor deposition.
 18. Themanufacturing method according to claim 15, wherein the step ofperforming the chemical vapor deposition is performed under conditionsthat temperature is greater than or equal to 600° C. and less than orequal to 700° C., pressure is greater than or equal to 20 Pa and lessthan or equal to 200 Pa, a flow rate of a SiH₄ gas is greater than orequal to 300 sccm and less than or equal to 3000 sccm, and a flow rateof an N₂ gas is greater than or equal to 0 sccm and less than or equalto 1000 sccm.
 19. The manufacturing method according to claim 15,wherein a single whisker of the group of whiskers has a width of 50 nmto 300 nm, a diameter of 100 nm to 400 nm, and a length of less than 800nm.
 20. The manufacturing method according to claim 15, wherein thesecond substrate is any one of an aluminosilicate glass substrate, abarium borosilicate glass substrate, an aluminoborosilicate glasssubstrate, a sapphire substrate, and a quartz substrate, wherein a groupof silicon whiskers is grown directly on the first substrate.
 21. Themanufacturing method according to claim 15, wherein a distance betweenthe second substrate and the first substrate is greater than or equal to1.0 cm and less than or equal to 3.0 cm.
 22. The manufacturing methodaccording to claim 15, wherein a distance between the second substrateand the first substrate and a distance between the second substrate andthe third substrate are each greater than or equal to 1.0 cm and lessthan or equal to 3.0 cm.
 23. The manufacturing method according to claim15, wherein each of a thickness of the first seed atom layer and athickness of the second seed atom layer is greater than or equal to 10nm and less than or equal to 1000 nm.
 24. A manufacturing method,comprising the steps of: placing a substrate comprising a first seedatom, a first substrate, and a substrate comprising a second seed atomso that a surface of the substrate comprising the first seed atom facesone surface of the first substrate, and a surface of the substratecomprising the second seed atom faces the other surface of the firstsubstrate, and introducing a gas containing silicon and performingchemical vapor deposition so as to grow a group of whiskers from thefirst substrate, wherein no seed atom layer is included with the firstsubstrate.
 25. The manufacturing method according to claim 24, whereinthe group of whiskers is formed on the one surface and the other surfaceof the first substrate.
 26. The manufacturing method according to claim24, wherein a reduced pressure chemical vapor deposition device is usedfor introducing the gas containing silicon and performing the chemicalvapor deposition so as to grow the group of whiskers from the firstsubstrate.
 27. The manufacturing method according to claim 24, whereinthe step of performing the chemical vapor deposition is performed underconditions that temperature is greater than or equal to 600° C. and lessthan or equal to 700° C., pressure is greater than or equal to 20 Pa andless than or equal to 200 Pa, a flow rate of a SiH₄ gas is greater thanor equal to 300 seem and less than or equal to 3000 seem, and a flowrate of an N₂ gas is greater than or equal to 0 seem and less than orequal to 1000 seem.
 28. The manufacturing method according to claim 24,wherein the chemical vapor deposition is reduced pressure chemical vapordeposition.
 29. The manufacturing method according to claim 24, whereina distance between the first substrate and the substrate comprising thefirst seed atom is greater than or equal to 1.0 cm and less than orequal to 3.0 cm.
 30. The manufacturing method according to claim 24,wherein a distance between the first substrate and the substratecomprising the first seed atom and a distance between the firstsubstrate and the substrate comprising the second seed atom are eachgreater than or equal to 1.0 cm and less than or equal to 3.0 cm.